Voltage regulator circuits function to take a varying input supply voltage and generate a stable output voltage. For example, the varying input supply voltage may comprise a battery supplied voltage and the stable output voltage is used to power analog and/or digital circuitry in a battery powered circuit application. The usable operating voltage and current overhead required by the voltage regulator circuit is a critical design consideration. The usable operating voltage is often referred to as the “drop-out” voltage, and this refers to the difference between the varying input supply voltage and the stable output voltage provided by the voltage regulator circuit. The smaller the “drop-out” voltage the better the system operation. Additionally, because the battery can supply only a finite amount of charge, it is important for the voltage regulator circuit to have as small a quiescent current as possible. The combination of a small “drop-out” voltage and small quiescent current ensures a more efficient and longer system operation from limited resource of a battery supply.
In view of the foregoing, there is considerable interest in the art in so-called low drop-out (LDO) voltage regulator circuits. Such regulators advantageously can maintain voltage regulation of the stable output voltage even when the level of the varying input supply approaches that stable output voltage. Maintenance of the stable output voltage is a challenge in the presence of varying load conditions. This is especially true when the load being supplied from the voltage regulator circuit includes digital circuitry. Those skilled in the art recognize that digital circuits are noisy and present a frequently changing load condition. The voltage regulator circuit must respond to those changing load conditions in generating the stable output voltage. However, voltage regulator circuits with low quiescent current characteristics tend to have poor transient response characteristics.
There exists a need in the art for a voltage regulator circuit, in particular of the low drop-out (LDO) type, which exhibits better transient response to varying load conditions.